In presence or intrusion detection systems it is known to use radiant energy or light beams to define a boundary of the space or region which is to be guarded or maintained under surveillance. Such a system, especially adapted for safeguard of industrial machines is disclosed in U.S. Pat. No. 3,970,846 granted July 20, 1976 to Schofield et al for "Presence Detecting System with Self-Checking" and assigned to the same assignee as this invention. The entire disclosure of this patent is hereby incorporated by reference into this application.
Certain industrial machines are difficult to safeguard to prevent injury to the operator without causing serious interference with the operation of the machine or restriction on the operator. For example, a metal bending machine such as a press brake, typically requires that the sheet metal workpiece be grasped by the operator at one portion while another portion is being formed in the die. For protection of the operator, there should be a barrier between the operator and the pinch-point in the die at a safe distance from the pinchpoint; however, the workpiece would have to extend through the barrier to be grasped by the operator and, furthermore, the workpiece must be allowed freedom of movement in the region of the barrier. A mechanical barrier cannot be used because it would interfere with movement of the workpiece during the forming operation. A barrier of the light beam type which forms a "light screen" or a "light curtain" has not heretofore provided an adequate solution. In such a light beam device, the workpiece would be detected as an intrusion through the light screen and the machine would be stopped. Muting of a light screen type of safety device during the down stroke of the slide of the machine does not afford continuous or a full time protection for the operator.